The metabolism of benzo(a)pyrene (BP) by human bronchus and pulmonary alveolar macrophages (PAM) from the same donor was studied to investigate whether the PAM might be useful as an indicator cell for the human bronchus. PAM were able to enzymatically to convert BP into metabolites which bound to cellular macromolecules. Negative correlation was observed between aryl hydrocarbon hydroxylase and binding of BP to cellular protein. A 9-fold interindividual variation was found in the binding to DNA. The majority of the metabolites of BP were water-soluble, while 7,8-dihydro-7,8-dihydroxy BP (7,8-diol), 9,10-dihydro-9,10-dihydroxy BP, triols and tetrols were the major metabolites in the ethyl extractable fraction. PAM metabolize BP into its proximate carcinogenic form, 7,8-diol, which is released into the extracellular space. No correlation of AHH activity and binding to DNA or protein between PAM and bronchus was found.